Because the hypothesis of surplus-value extraction is easy to see and experience: find out how much work it takes to produce your salary and benefits in revenue, and then start refusing to work more than that. You will be fired. Therefore, your boss is extracting surplus value from you.
If you are a right-wing proprietarian, you might not object to that, on grounds you consented. If you're not that, you will feel exploited. The fact that the USA has the OECD's worst governmental working-condition laws that completely fail to make the extraction process feel more like a normal, bearable part of human life doesn't help.
I don't understand how this relates to my previous comment.
As for your rant about surplus-value extraction: Presumably your boss isn't just taking your surplus for free; they're providing things that helps you work more efficiently. For example, if you're a steelworker, your employer provides a very expensive steel mill; without access to that equipment, your steelmaking skills are fairly useless.
In industries that have little capital requirement, like software development or web design, the things an employer might bring to the table to enhance their employees' created value include: A product vision, an already-existing codebase, brand or website, a talented team, support services such as marketing, accounting, legal...
Also, working for an employer lowers risk for employees who work on uncertain ventures. If you spend half a year developing a new product for an employer, you still get paid for those six months even if it's a total flop and nobody ever buys it. But if you'd built it on your own time and bootstrapped it into a startup instead, yes, you'd keep the entire profit if it went well -- but you'd get nothing (financially) from those six months of work if it flopped. This "insurance" against product flops is part of what the employer's portion of your created value pays for.
If you feel you're being exploited -- your employer is taking too much of the value you're creating -- then you're free to negotiate with them, change employers, change industries, or build your own startup.
If you are a right-wing proprietarian, you might not object to that, on grounds you consented. If you're not that, you will feel exploited. The fact that the USA has the OECD's worst governmental working-condition laws that completely fail to make the extraction process feel more like a normal, bearable part of human life doesn't help.